Holographic security markers are currently incorporated into a wide range of products which have high intrinsic value or which contain personal or commercially sensitive information. Credit cards and many passports include holographic panels, as do many currency notes. Typically those holographic panels are surface holograms, otherwise known as relief holograms or holographic optical elements (HOEs) embossed into a surface of a portion, typically a metal foil portion, of the product which needs to be certified as genuine. Such HOEs are formed by the formation of a surface diffraction grating on the foil. Typically the surface diffraction grating is embossed by pressing onto the foil a master ‘negative’ formed from a material harder than the metal of the foil. The press used is similar to a printing press.
It is known that volume holograms can include far more readable data than surface holograms or HOEs. Their use is however limited because the higher cost of creating volume holographic images currently outweighs the advantages. Volume holograms have been proposed which replay in the visible range. Volume holograms have also been proposed which replay in the invisible range, such as the infrared (IR) range. Such an image is only properly displayed when interrogated with an appropriate beam which matches the recording beam in frequency and incidence angle, although it is known how to move the replay frequency to another frequency, typically by swelling or contracting the substrate in which the hologram is recorded. WO-A-2008/045625 discloses the creation of a holographic security panel of a high value article such as a mobile telephone in which two volume holographic images are recorded in the same portion of the telephone casing. One image is in the visible range of the spectrum and the other is in the invisible portion of the spectrum. The volume holograms disclosed in WO-A-2008/045625 are however transmission holograms which when replayed generate an image in space above or remote from the recorded holographic film.
Even if two volume holographic images are recorded in the same security panel as proposed above, they must be recorded separately, and the accuracy of the register of the two holographic images is therefore just as good as the accuracy of the set-up of the initial recordings. In many security markers, including those used to verify the genuine nature of bank notes, one valuable aspect of the security guarantee is the accuracy of alignment of two or more elements of the printed or holographic security markings. This invention seeks to provide a security marker and system in which a far higher degree of security is established by the accurate alignment of portions of a recorded volume hologram which have mutually different replay frequencies, optionally together with an equally accurate alignment of those portion with elements of a surface relief hologram (HOE) recording.